DocBook XML - Validating

Hello 

Could someone please provide me with instructions for
using the XERCES-C/J for validating an XML document
against the DocBook DTD (need to plug up/set path the
catalog file)? or If any free parser (XML validating)
software will support following requirements:
conform to W3C XML 1.0 standards, Unicode, namespaces,
catalog file...

Please suggest.

Thanks
Tom

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